Component for a high-temperature steam turbine , and temperature steam turbine

ABSTRACT

A component for a high-temperature steam turbine which operates at temperatures above 600° C., especially above 700° C., is formed of a nickel-based alloy. The negative influence of oxidation of the component which is induced by the superheated steam is prevented by the alloy which is used, having the following composition (in % by weight): 
     
       
         
               
               
               
             
                   
                   
               
                   
                 C: 
                 ≦0.2 
               
                   
                 Si: 
                 ≦1.0 
               
                   
                 Mn: 
                 ≦1.0 
               
                   
                 Cr: 
                 22.0-25.0 
               
                   
                 Co: 
                 15.0-25.0 
               
                   
                 Mo: 
                 ≦3.0 
               
                   
                 Nb: 
                 ≦2.0 
               
                   
                 Al: 
                 1.0-3.0 
               
                   
                 Ti: 
                 2.0-4.0 
               
                   
                 Fe: 
                 ≦2.0 
               
                   
                 Zr: 
                 ≦0.2 
               
                   
                 B: 
                  ≦0.05 
               
                   
                 Ni: 
                 remainder.

This application claims priority under 35 U.S.C. §119 to Swiss application no. 01638/08, filed 13 Oct. 2008, the entirety of which is incorporated by reference herein.

BACKGROUND

1. Field of Endeavor

The present invention refers to the field of steam turbines. It relates to a component for a high-temperature steam turbine and to a high-temperature steam turbine.

2. Brief Description of the Related Art

Modern steam turbines operate at very high temperatures in order to improve the efficiency. At temperatures above 600° C., and especially above 700° C., many components of such a high-temperature steam turbine are produced from nickel-based alloys in order to achieve the necessary high-temperature resistance (creep, fatigue, etc.).

At high operating temperatures, the materials which are used (and also the nickel-based alloys) oxidize if they are exposed to the superheated steam. This is especially the case with turbine blades in which the oxidation leads to

-   -   a loss of loadable area increasing the operation-related load;     -   the oxidation creating “micro-notches” along the grain         boundaries, which increase the risk of fatigue crack         development;     -   a surface roughness being created due to the formation of         irregular oxide layers, and a separation of oxides from the         surface occurring (both of which effects lead to a roughening of         the surface which reduces the aerodynamic efficiency of the         turbine).

A steam turbine power plant with operating temperatures of above 650° C. is known from U.S. Pat. No. 7,238,005, which is characterized by the use of selected nickel-based alloys instead of austenitic, heat-resistant steels for specific components of the turbines, such as casings or blades for example, in order to reduce the thermal stresses during load changing. The alloys which come into question in this case have different portions of Cr, Co and Mo, in fact have either a Cr portion of ≦21% by weight, for example 11.0 to 20.0, or 17.0 to 21.0% by weight, or a Cr portion of 19.0 to 24.0% by weight but a Co portion of ≦15% by weight, or a Cr portion of 19.0 to 25.0% by weight but no Co portion and an Mo portion of ≦2.0% by weight.

The question of grain boundary oxidation and the fatigue crack development which is associated with it, and also questions of oxidation-related surface roughness, play no role in the case of the solution of U.S. Pat. No. 7,238,005.

SUMMARY

One of numerous aspects of the present invention includes improving the highly loadable components of a superheated steam turbine, which are exposed to the superheated steam, with regard to the possible oxidation and its adverse effects, and also to a corresponding superheated steam turbine.

Another aspect includes that the component is produced from a special alloy which, on account of the selection of the alloy elements, especially of the chromium portion, combines the high strength at high operating temperatures of nickel-based alloys with the high resistance to grain boundary oxidation, to oxidation of the surface in general, and to the separation of surface oxides.

The alloy which is used has the following composition (in % by weight) according to an exemplary embodiment of the invention:

C: ≦0.2 Si: ≦1.0 Mn: ≦1.0 Cr: 22.0-25.0 Co: 15.0-25.0 Mo: ≦3.0 Nb: ≦2.0 Al: 1.0-3.0 Ti: 2.0-4.0 Fe: ≦2.0 Zr: ≦0.2 B:  ≦0.05 Ni: remainder.

According to a development of the invention, the portion of Cr is limited to the range of between 23.0 and 25.0 (% by weight).

The component is preferably a blade of the steam turbine.

An exemplary high-temperature steam turbine according to the invention is characterized in that the component consists of a nickel-based alloy which has the following composition (in % by weight):

C: ≦0.2 Si: ≦1.0 Mn: ≦1.0 Cr: 22.0-25.0 Co: 15.0-25.0 Mo: ≦3.0 Nb: ≦2.0 Al: 1.0-3.0 Ti: 2.0-4.0 Fe: ≦2.0 Zr: ≦0.2 B:  ≦0.05 Ni: remainder.

According to a development, the portion of Cr in this case is limited to the range of between 23.0 and 25.0 (% by weight). The component is especially a blade of the high-temperature steam turbine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is subsequently to be explained in more detail in conjunction with the drawing. In the drawing

FIG. 1 shows a diagram of the mean resistance to creep fracture after 1000 hours over the temperature of a commercially available alloy according to the invention (Nimonic 101®) in comparison to a previously used commercially available alloy (Waspaloy®) and

FIG. 2 shows a diagram of the mean tensile strength over the temperature of the two alloys from FIG. 1 in comparison.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

According to principles the present invention, a nickel-based alloy is used for the components, especially blades (rotor blades and/or stator blades), of a high-temperature steam turbine, which are loaded with high steam temperatures of 600° C. and more, which nickel-based alloy is characterized by a comparatively high content of Cr and Co and has the following composition (in % by weight):

C: ≦0.2 Si: ≦1.0 Mn: ≦1.0 Cr: 22.0-25.0 Co: 15.0-25.0 Mo: ≦3.0 Nb: ≦2.0 Al: 1.0-3.0 Ti: 2.0-4.0 Fe: ≦2.0 Zr: ≦0.2 B:  ≦0.05 Ni: remainder.

As a result of this, components are realized which are characterized by high strength at the operating temperatures and have a large resistance to oxidation on the grain boundaries and also on the surfaces in general, and are not prone to separation of oxide layers from the surfaces.

Instead of a Cr content of 22.0-25.0% by weight, a restricted range of 23.0-25.0% by weight can advantageously be used.

A commercially available nickel-based alloy which lies within the composition spectrum according to the invention is known by the trade name Nimonic 101®. This known nickel-based alloy contains 24.2% by weight of Cr, 19.7% by weight of Co, 1.5% by weight of Mo, 3.0% by weight of Ti, 1.4% by weight of Al, 1.0% by weight of Nb, and 48.0% by weight of Ni, and has not yet been used previously for steam turbine components.

By a comparison with another commercially available nickel-based alloy, that is to say Waspaloy®, which only contains 18.0 to 21.0% by weight of Cr and 12.0 to 15.0% by weight of Co, it is clear that even at the high temperatures above 600° C. there are significant advantages for the alloys according to the invention.

Therefore, FIG. 1 shows that the mean resistance to creep fracture at 1000 h (Rm, 1000 h) of Nimonic 101® in the interesting temperature range above 600-650° C. is higher by up to 16% than the corresponding value for Waspaloy®.

In the same way, according to FIG. 2 the mean tensile strength (Rm) of Nimonic 101® above 650° C. is slightly higher than that of Waspaloy®.

In all, the following advantages are created by the invention:

-   -   a mechanical failure of the blades is excluded (long service         life);     -   a loss of aerodynamic efficiency on account of increased surface         roughness is avoided.

While the invention has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents. 

1. A component for a high-temperature steam turbine which operates at temperatures above 600° C., wherein the component consists of a nickel-based alloy having the following composition (in % by weight): C: ≦0.2 Si: ≦1.0 Mn: ≦1.0 Cr: 22.0-25.0 Co: 15.0-25.0 Mo: ≦3.0 Nb: ≦2.0 Al: 1.0-3.0 Ti: 2.0-4.0 Fe: ≦2.0 Zr: ≦0.2 B:  ≦0.05 Ni: remainder.


2. The component as claimed in claim 1, wherein Cr is between 23.0% and 25.0% by weight.
 3. The component as claimed in claim 1, wherein the component is a blade of the high-temperature steam turbine.
 4. A high-temperature steam turbine for operation at temperatures above 600° C., wherein the high-temperature steam turbine comprises a component which is exposed to superheated steam during operation of the turbine, wherein the component consists of a nickel-based alloy having the following composition (in % by weight): C: ≦0.2 Si: ≦1.0 Mn: ≦1.0 Cr: 22.0-25.0 Co: 15.0-25.0 Mo: ≦3.0 Nb: ≦2.0 Al: 1.0-3.0 Ti: 2.0-4.0 Fe: ≦2.0 Zr: ≦0.2 B:  ≦0.05 Ni: remainder.


5. The high-temperature steam turbine as claimed in claim 4, wherein Cr is between 23.0% and 25.0% by weight.
 6. The high-temperature steam turbine as claimed in claim 4 wherein the component is a blade of the high-temperature steam turbine. 